The widely usage of halogen and nitroaromatic solvents, including perchloroethylene (PCE), trichloroethylene (TCE), and nitrobenzene, has led to the continued release of such toxic organic compounds into the environment during industrialization. As persistent pollutant, those contaminants are quite stable under natural conditions, and easily accumulated in aquatic environment for years, thus causing incredible effects on human beings and the aquatic ecological environment.
Over the years, many methods have been developed for degrading those contaminants in soil and water, some relating to in situ treatment and others that relate to ex situ treatment. For instance, anaerobic biodegradation, advanced oxidation, thermal desorption, activated carbon adsorption, zero-valent iron reduction reaction, and metal catalyst reduction reaction with hydrogen as reductant using Fe, Ni, Pt, Pd, Rh, etc. However, the above treatment methods have some drawbacks.
For the anaerobic biodegradation of reductive dechlorination, dissolved oxygen inhibits or even kills the dechlorinated bacteria, thus, a strict anaerobic environment is required. Also, cobalamin is a necessary nutritional factor required. Moreover, the types of electron donor, pH, or temperature etc. also affect the biodegradation efficiency. The long processing time is another drawback for biodegradation.
For advanced oxidation and thermal desorption, the high cost would be the main limiting factor. As is to activated carbon adsorption, in fact, the method cannot completely degrade the contaminants, but only transfer the contaminants from aqueous phase to solid phase. However, hydroxide or carbonate precipitation forms in the surface of zero-valent iron in the process of reducing dechlorination, thereby reducing its reductive dechlorination effect.
For metal catalyst reduction reaction, in order to achieve efficient decontamination, suitable pH condition, high operating temperature is usually required. The cost for operation is expensive, let alone the high price of catalyst. When used in situ treatment, the metal catalysts would be poisoned by the SO32− and HS− in groundwater, disabling the activity of the metal catalysts. Moreover, metal catalysts are expensive and are also easy to cause secondary contamination, thereby limiting its application.
So currently, it is still very significant to develop more economical and effective method to treat contaminated groundwater or soil.